Rockwell Automation T8xxx Trusted Communications Interface - Manual (Page 41)

Trusted
TM
Communication Interface T8151B
Issue 21 Apr 10 PD-T8151B 41
3.10. Modbus Slave Timing and Throughput Rules
If the Trusted TM system is Modbus Slave, then it has no control over the messages received from the
Master. If the Trusted TM system is delaying more than expected between messages, it is possible that
insufficient time is allowed for communications processing between the Communication Interface and
the Processor. It is possible to allocate some time to background tasks like this by specifying a sleep
period between scans in the system configuration.
This is especially a problem with systems that have no native I/O, e.g. bridged systems, and therefore
do not have communications processing time allocated.
In the System Configuration processor parameters shown below, set the sleep period to approximately
32ms longer than the ‘current’ scan time seen in the Toolset debugger window. This will provide some
time on most scans for communications tasks, and will even out the scan time. It will also lengthen the
scan time, so ensure that process safety times are not compromised and the maximum scan time and
I/O module watchdog times are adjusted proportionately to the change in average scan time.
Figure 17 ISaGRAF Sleep Period
For information on message timing, refer to section 3.9.